The present invention relates to an electric lamp with high efficiency, good color rendering, and high lamp lumen maintenance. It finds particular application in connection with a ceramic metal halide lamp with halides of barium or strontium in the fill and will be described with particular reference thereto.
Discharge lamps produce light by ionizing a vapor fill material such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
Ceramic metal halide lamps have been developed with efficiencies in the range of about 90-100 lumens per watt (LPW), color rendering indices Ra of 85-95, or higher, and lumen maintenance values of 80%, or higher, and color temperatures of between about 2600 and 4000K at wall loadings of from about 20 to 50 W/cm2. However, premature failure of the lamps may occur due to blackening of the discharge vessel walls. The blackening is due to tungsten transferred from the filament to the wall. The presence of oxygen and/or water vapor in the lamp atmosphere has been found to contribute to the wall blackening. Water vapor is particularly harmful because even trace amounts increase the evaporation of the tungsten filament coil by means of the well-known “water cycle.” In the water cycle, the temperature of the tungsten coil is thermally sufficient to decompose water vapor into hydrogen and oxygen. The resulting oxygen reacts with the tungsten from the coil to form volatile oxides, which migrate to cool parts of the lamp and condense. These oxide deposits are reduced by the gaseous hydrogen to yield black metallic tungsten and reformed water, which causes the cycle to repeat.
Tungsten halogen lamps, which comprise a hermetically sealed, light transmissive discharge vessel enclosing a tungsten filament and containing a fill comprising a halide or halogen gas are widely used in a variety of applications. Some of these lamps operate on a tungsten halogen cycle which is a regenerative, continuous process in which a halogen-containing tungsten compound is produced when the halide combines chemically with particles of tungsten evaporating from the incandescent tungsten filament. Subsequent thermal decomposition of these so-formed halogen-containing tungsten compounds at the filament returns the tungsten particles back to the filament. Halogen compounds used for the fill include bromine and bromides, such as hydrogen bromide, methyl bromide, dibromomethane, and bromoform. Lamps that operate at low wall loadings (WL), e.g., below about 30 W/cm2, and thus low temperatures, i.e. below about 200° C. interior wall temperatures, generally do not support the tungsten halogen cycle. Additionally, if WL is too low then the halide temperature tends to be too low leading to reduced halide vapor pressure and reduced performance.
It has been proposed to incorporate of a calcium oxide or tungsten oxide dispenser in the discharge vessel, as disclosed, for example in WO 99/53522 and WO 99/53523 to Koninklijke Philips Electronics N.V. U.S. Pat. No. 6,844,676 to Alderman, et al. discloses an arc tube fill comprising metallic mercury, a mixture of noble gases and, optionally, radioactive 85Kr, and a salt mixture such as a mixture composed of sodium iodide, calcium iodide, thallium iodide, and several rare earth iodides.
The exemplary embodiment provides a new and improved metal halide lamp capable of operating at high or low power which has a high efficiency and good color rendering.